In recent years, as television receivers have been increased in size and enhanced in image quality, enhancement in performance of a luminance signal/chrominance signal separation device for separating a luminance signal and a chrominance signal from a composite video signal has increasingly become important.
Hereinafter, a conventional luminance signal/chrominance signal separation device will be described.
FIG. 13 is a block diagram illustrating a conventional luminance signal/chrominance signal separation device.
In FIG. 13, reference numerals 601 and 602 denote line memories each having a storage capacity for one line of a video signal; numerals 603, 604, and 605 denote filters which pass predetermined band components; numerals 606 and 607 denote inversion circuits for inverting input signals; numerals 608, 609, 610, 615, 618, 619, and 620 denote minimum value circuits each for outputting a minimum value between two input signals; numerals 611, 612, 613, 614, and 621 denote maximum value circuits each for outputting a maximum value between two input signals; numeral 616 denotes an addition circuit for adding two input signals; numeral 617 denotes a one-half circuit for outputting a one-half value of an input signal; numeral 623 denotes a three-line chrominance separation circuit comprising the above-mentioned components 606˜621; and numeral 622 denotes a subtracter for subtracting an output signal of the three-line chrominance separation circuit 623 from an output signal of the line memory 601.
The line memories 601 and 602 are connected in series, and a composite video signal S601 from which a luminance signal and a chrominance signal are to be separated is input to the line memory 601. The composite video signal S601 is input to the filter 603, an output signal S602 from the line memory 601 is input to the filter 604, and an output signal S603 from the line memory 602 is input to the filter 605.
In the three-line chrominance separation circuit 623, output signals S604 and S606 from the filters 603 and 605 are input to the inversion circuits 606 and 607, respectively. An output signal S607 from the inversion circuit 606 and an output signal S605 from the filter 604 are input to the minimum value circuit 608 and to the maximum value circuit 612. An output signal S608 from the inversion circuit 607 and the output signal S605 from the filter 604 are input to the minimum value circuit 609 and to the maximum value circuit 613.
Output signals S609 and S610 from the minimum value circuits 608 and 609 are input to the minimum value circuit 610 and to the maximum value circuit 611, and output signals S613 and S614 from the maximum value circuits 612 and 613 are input to the maximum value circuit 614 and to the minimum value circuit 615, respectively.
An output signal S611 from the minimum value circuit 610 and an output signal S615 from the maximum value circuit 614 are input to the adder 616.
An output signal S617 from the adder 616 is input to the one-half circuit 617. An output signal S612 from the maximum value circuit 611 and an output signal S618 from the one-half circuit 617 are input to the minimum value circuit 618. An output signal S616 from the minimum value circuit 615 and the output signal S618 from the one-half circuit 617 are input to the minimum value circuit 619. The output signal S612 from the maximum value circuit 611 and the output signal S616 from the minimum value circuit 615 are input to the minimum value circuit 620. Output signals S619, S620, and S621 from the minimum value circuits 618, 619, and 620 are input to the maximum value circuit 621.
Further, the output signal S602 from the line memory 601 and an output signal S622 from the maximum value circuit 621 of the three-line chrominance separation circuit 623 are input to the subtracter 622.
Hereinafter, the operation of the luminance signal/chrominance signal separation device so constituted will be described.
A composite video signal S601, which is obtained by A/D converting an NTSC signal, is input to the line memories 601 and 602, thereby obtaining a composite video signal S602 that is delayed by one line in the line memory 601, and a composite video signal S603 that is delayed by another one line in the line memory 601. Then, these composite video signals S601, S602, and S603 are input to the filters 603, 604, and 605, respectively, thereby obtaining composite video signals S604, S605, and S606 that are band-restricted by these filters 603, 604, and 605, respectively.
Since the phase of the chrominance signal is inverted at 180° for every line, the composite video signal S605 is assumed as a center line, and the phases of the composite video signals S604 and S606 in lines before and after the center line are inverted by the inversion circuits 606 and 607 to obtain signals S607 and S608, thereby matching the phases of the chrominance signals among the three lines. Thereafter, an intermediate value between a maximum value and a minimum value among the three input signals S607, S605, and S608 is detected utilizing correlation in the vertical direction among the three lines, by a section comprising the minimum value circuits 608, 609, 610, 615, 618, 619, and 620, the maximum value circuits 611, 612, 613, 614, and 621, the adder 616, and the one-half circuit 617, which are included in the chrominance separation circuit 623, and this intermediate value is regarded as a chrominance signal to be output as an output signal S622. Therefore, when there is correlation among the chrominance components included in the composite video signals S601, S602, and S603, the chrominance signal can be correctly separated. Then, the chrominance signal S622 is subtracted from the composite video signal S602 by the subtracter 622 to obtain a luminance signal S623.
The conventional luminance signal/chrominance signal separation device constituted as described above can correctly separate the luminance signal and the chrominance signal from the composite video signal when there is line correlation in the vertical direction. However, when a luminance signal that is imaged as diagonal stripes is input, the line correlation in the vertical direction is lowered.
FIG. 14 is a diagram illustrating timing charts of the respective signals in a case where diagonal stripes are input to the conventional luminance signal/chrominance signal separation device.
With reference to FIG. 14, S601 indicates an inputted composite video signal, S602 indicates the composite video signal that is delayed by one line, and S603 indicates the composite video signal that is delayed by two lines, in a state where diagonal stripes of a luminance signal having a frequency component in the vicinity of 3.58 MHz are input. Further, S604, S605, and S606 indicate band-restricted video signals which are obtained by filtering the composite video signals S601, S602, and S603 so as to remove low-frequency components thereof, and pass only signal components having a frequency band in the vicinity of 3.58 MHz.
Further, S607 and S608 indicate video signals which are obtained by inverting the signals S604 and S606 in the lines above and beneath the signal S605 in the center line to match the phases of these signals. Furthermore, S609˜S621 indicate results obtained in the middle of processing, more specifically, which are obtained by passing the signals S605, S607, and S608 through the maximum value circuits and the minimum value circuits in the three-line chrominance separation circuit 623, and S622 indicates an output result of a chrominance signal that is detected by selecting a maximum value and a minimum value.
In this way, when the diagonal stripes are input, signal components are output from the output terminal of the chrominance signal S622 of the three-line chrominance separation circuit 623 although the original chrominance signal is not output. That is, the inputted luminance signal components leak into the chrominance signal components to be output, whereby cross color is generated in the position where the diagonal stripes are input. Further, since the luminance signal S623 is obtained by subtracting the chrominance signal S622 from the composite video signal S602, the leakage of the luminance signal components into the chrominance signal components causes degraded resolution.